Method for recovering rare-earth elements from a solid mixture containing a halophosphate and a compound of one or more rare-earth elements

ABSTRACT

A method is described for recovering rare earth elements from a solid mixture including a halophosphate and at least one compound of one or more rare earth elements. The method includes: (a) acid etching the mixture; (b) adding a base to bring the pH back up to a value of at least 1.5; (c) etching the solid from step (b) with a solution of soda or potash; (d) acid etching the solid from step (c) until a pH of less than 7 is obtained, resulting in a solid phase and a liquid phase including at least one rare earth salt, and separating the solid phase from the liquid phase.

The invention relates to a method of recovering rare-earth elements froma solid mixture containing a halophosphate and a compound of one or morerare-earth elements.

The market for energy-saving lamps is now expanding rapidly. It is knownthat lighting is in fact a major item in the energy bill ofindustrialized countries and that using these lamps instead ofincandescent lamps will make it possible to reduce its level.

Government directives issued in several countries will reinforce thistrend toward energy-saving lamps.

Moreover, development of lamps of this type makes it imperative tomanage their recovery and recycling, taking into account not only thepresence of mercury, but also of the metals included in theircomposition.

At present, almost all the materials (glass, mercury, metals, etc.)resulting from the processing of lamps of this type are recycled,reaching a level of recycling above 90%. Only the fluorescent powders,representing 1-4 wt. % of the lamps, are currently buried at approvedlandfill sites, after removal of mercury.

These fluorescent powders comprise one or more luminophores as well asadditives for improving the performance of the lamps (alumina, calciumphosphate or borate etc.).

Now, the luminophore compounds present in these powders are materialswhich are expensive to produce, and which contain expensive rareelements, such as rare-earth elements.

There is therefore a great need for a method that would allow thevarious components of these powders to be separated, so that they canthen be reprocessed separately.

The method of the invention meets this need.

With this aim, the method of the invention is a method of recoveringrare-earth elements from a solid mixture containing at least onehalophosphate and at least one compound of one or more rare-earthelements, and it is characterized in that it comprises the followingsteps:

(a) acid attack is carried out on said mixture in a liquid medium;

(b) a base is added to the medium obtained at the end of step (a) so asto raise the pH of said medium to a value of at least 1.5, whereby afirst solid phase is obtained comprising one or more rare-earth elementsat least partly in the form of phosphate and a first liquid phasecomprising at least one alkaline-earth element of the halophosphate andthe first solid phase is separated from the first liquid phase;

(c) the solid resulting from step (b) is attacked with a strong base,whereby a second solid phase is obtained comprising one or morerare-earth elements at least in the form of hydroxide and a secondliquid phase comprising a phosphate of the cation of the strong base andthe second solid phase is separated from the second liquid phase;

(d) acid attack of the solid resulting from step (c) is carried out inconditions such that the pH of the reaction mixture is below 7, giving athird solid phase and a third liquid phase comprising at least onerare-earth salt and the third solid phase is separated from the thirdliquid phase.

The method of the invention is a simplified method which gives a highyield in recovery of the rare-earth elements, which can notably be atleast 75% or even at least 80%.

Moreover, according to a particular embodiment, this method gives riseto a solid effluent that can be utilized and to a liquid effluent thatis acceptable, taking into account the environmental constraints.

Other characteristics, details and advantages of the invention willbecome clearer on reading the description given hereunder, referring tothe appended drawing in which:

the single FIGURE is a block diagram of the method of the invention.

For the present description, “rare-earth elements” means the elements ofthe group consisting of yttrium and the elements of the periodic tablewith atomic number between 57 and 71 inclusive.

It should also be noted that hereinafter, unless stated otherwise, inall the ranges or limits of values that are given, the values at thelimits of the range are included, the ranges or limits of values thusdefined therefore covering any value at least equal to or greater thanthe lower limit and/or at most equal to or less than the upper limit.

The method of recovery of the invention starts from a mixture in thesolid form which notably contains at least one halophosphate and atleast one compound of one or more rare-earth elements.

The method of the invention can be applied particularly well to amixture resulting from processing for recovery or recycling of lamps,for example fluorescent tubes, linear, compact, trichromatic or halogenlamps, said processing having already recovered, at least partly,materials such as glass, mercury or other metals.

This mixture is generally in the form of a powder whose granulometry canvary notably between 1 μm and 10 μm, more particularly between 3 μm and10 μm and whose density can be for example between 3 and 5.

A first type of halophosphate present in the starting mixture is aluminophore used in the lamps for its white color of emission. It isgenerally of a compound of the apatite type, i.e. a compound which is amixture of a calcium phosphate and of another salt of this element andwhich can also comprise halogens such as fluorine or chlorine. We maymention as examples hydroxyapatite Ca₁₀(PO₄)₆(OH)₂, chloroapatiteCa₃(PO₄)₂CaCl₂, fluoroapatite Ca₅(PO₄)₃F. As is known, the compounds ofthis type can further comprise another alkaline-earth element such asstrontium partially replacing calcium, doping elements of the transitionelement type such as antimony or manganese or of the rare earth type. Asan example, we may mention a compound represented by the general formula(Sr,Ca)₁₀(PO₄)₆(Cl,F)₂:Sb³⁺,Mn²⁺.

As another type of halophosphate, we may mention the products of thephosphate type comprising a halogen, an alkaline-earth element and arare-earth element such as europium for example. We may mention, as anexample of a compound of this type, the compound of formulaSr₅(PO₄)₃Cl:Eu²⁺. These halophosphates are generally used for their bluecolor of emission.

The solid starting mixture further comprises a compound of one or morerare-earth elements.

These compounds can be of various kinds. They can be selected moreparticularly from phosphates, alkaline-earth aluminates, borates,vanadates and oxides of rare-earth elements.

For the phosphates, we may mention more particularly the phosphates ofcerium or of terbium or of a combination of these two rare-earthelements. They can also be phosphates of lanthanum in combination withat least one of the two aforementioned rare-earth elements and they canalso be quite particularly phosphates of lanthanum, of cerium and ofterbium. These phosphates are generally orthophosphates, which can berepresented by the general formula LnPO₄, with Ln denoting at least onerare-earth element such as lanthanum, cerium and terbium. They arenotably luminophores used for their emission in the green.

For the aluminates, they are alkaline-earth aluminates, thealkaline-earth metal being more particularly magnesium, barium, calciumor strontium, alone or in combination. They are generally products usedin lamps for their emission in the blue or the green.

The rare-earth element, which can be present as element of the matrix oras dopant, can notably be cerium, terbium, europium, neodymium anddysprosium, these elements being used alone or in combination.

Thus, barium can be partially substituted with at least one elementselected from strontium, calcium or rare-earth elements other thaneuropium. Moreover, magnesium can be partially substituted with at leastone element selected from zinc or manganese or cobalt. Finally, aluminumcan also be partially substituted with at least one element selectedfrom gallium, scandium, boron, germanium or silicon.

Purely as an example, we may mention aluminates of the followingformulas: BaMgAl₁₀O₁₇:Eu²⁺ or (Ce,Tb)MgAl₁₁O₁₉.

The borates of rare-earth elements can be orthoborates of formula LnBO₃,Ln representing at least one rare-earth element or oxyborates ofrare-earth elements of formula Ln₃BO₆. It is known that in these boratesthe rare-earth element can be present as element of the borate matrix,in this case the rare-earth element can notably be lanthanum, lutecium,yttrium and gadolinium or a combination of at least two of theserare-earth elements but also as a doping element. In the latter case,the rare-earth element can be more particularly europium, samarium,thulium and ytterbium, these elements being used alone or incombination. The doping rare-earth element can also be present incombination with a second dopant such as bismuth, lead, antimony,chromium and iron.

As an example of borate, we may mention a product of formula:(Gd,Mg)B₅O₁₀:Ce³⁺,Tb³⁺.

The rare-earth elements can also be present in the form of vanadates,doped with a rare-earth element such as europium. We may thus mentionthe compounds YVO₄:Eu or Y(P,V)O₄:Eu.

Finally, the compound of rare-earth element of the starting mixture canbe a rare-earth oxide of general formula Ln₂O₃. We may mention moreparticularly yttrium oxide (Y₂O₃) or gadolinium oxide (Gd₂O₃) or themixed oxide of yttrium and gadolinium ((Y,Gd)₂O₃). These oxides ofyttrium and/or of gadolinium are generally doped with europium and theycan optionally contain other additional elements selected from therare-earth elements other than europium, gadolinium and yttrium. We maynotably mention terbium, samarium or lanthanum. Such oxides aregenerally used in lamps for their red emission.

The solid starting mixture can moreover contain compounds such asalumina Al₂O₃, silica SiO₂, calcium phosphates and other residuesresulting from processing of the lamps such as metals such as mercury orplastics or glass debris.

The relative proportions of halophosphate and of compounds of rare-earthelements can vary widely depending notably on the type of lamp fromwhich the mixture to be treated was obtained. The method of theinvention can be applied quite particularly to mixtures that have acontent of at most about 80 wt. % of halophosphate, but this value isnot critical and is only given as an example.

The various steps of the method of the invention will now be describedin detail and reference may be made, in the rest of the description, tothe appended drawing in which these steps are shown schematically.

The first step of the method of the invention, step (a), is acid attackof the solid mixture, which has just been described.

This attack is carried out in a liquid medium. Thus, a dispersion of thesolid mixture in water can be prepared and the resultant dispersion isbrought in contact with an acid. This can be carried out in a stirredreactor containing the dispersion, introducing a solution of acid intothe reactor.

The acid is preferably a strong acid such as nitric acid or hydrochloricacid.

Acid attack is preferably carried out hot, for example at a temperaturebetween 40° C. and 80° C.

Generally attack is carried out until the pH in the reaction mixture isless than or equal to 1.

Once the pH is reached after adding the necessary amount of acid, it maybe advantageous to carry out maturation of the reaction mixture byholding it in the same conditions of pH and of temperature as at the endof attack, optionally with stirring.

The second step of the method, step (b), comprises adding a base to themedium obtained at the end of step (a), optionally holding the medium atthe same temperature that it was at before introduction of the base.

It is preferable to use a base of the alkali-metal hydroxide type suchas soda. The base is added in such a way that the pH of the reactionmixture rises to a pH of at least 1.5, more particularly of at least 2.

After adding the base and, preferably, cooling the reaction mixture, thephases of the reaction mixture are separated by any suitable method ofsolid/liquid separation, for example filtration, centrifugation,decanting.

At the end of this separation, a first solid phase and a first liquidphase are collected. Washing of the solid phase with water can becarried out.

The first solid phase comprises the rare-earth element or elements ofthe solid starting mixture. This rare-earth element or these rare-earthelements are at least partly in the form of phosphate, notablyorthophosphate. The solid phase can in fact contain one or morerare-earth elements in another form, for example in the form ofaluminates and, optionally and in residual amounts, borates or oxides,notably depending on the composition of the solid starting mixture,these forms corresponding to rare-earth compounds that were not attackedduring the first step of acid attack. Finally, this solid phase can inaddition contain other compounds such as alumina or silica.

The first liquid phase comprises at least one alkaline-earth elementwhich is the alkaline earth of the starting halophosphate, notablycalcium or strontium in the form of a solution, generally aqueous, ofthe salt of the acid used during acid attack.

This liquid phase can either be removed as liquid effluent from theprocess, or it can be further treated subsequently according to avariant of the method which will be described later.

The next step of the method of the invention, step (c), is attack of thesolid (first solid phase) resulting from step (b) with a strong base,which more particularly can be a solution of soda or of potash.

This attack is carried out by bringing the strong base in the form ofsolution in contact with the solid, with stirring. The mixture obtainedis heated to a temperature of at least 100° C., more particularly of atleast 120° C., for a time that can vary between 30 min and 2 h forexample, said time being shorter, the higher the temperature of themixture.

At the end of attack, it is possible to dilute the reaction mixture byadding water to lower its viscosity, if necessary.

The medium is then submitted to separation of the solid and liquidphases, once again by any known means, for example by filtration.

At the end of this separation, a second solid phase and a second liquidphase are collected. Washing of the solid phase with water can becarried out.

This second solid phase comprises one or more rare-earth elements, atleast a proportion of which is in the form of hydroxide Ln(OH)₃, Lnrepresenting at least one rare-earth element.

Still notably depending on the composition of the solid startingmixture, the solid phase can contain one or more rare-earth elements inanother form, for example in the form of aluminates and, optionally andin residual amounts, borates or phosphates corresponding to rare-earthcompounds that were not attacked during the first step of acid attack,as well as other compounds such as alumina or silica.

The second liquid phase comprises compounds such as phosphate of thecation of the strong base used for the attack in step (c), for examplesuch as sodium phosphate or potassium phosphate. It can optionallycontain silicate of this cation such as sodium silicate or potassiumsilicate.

This liquid phase can either be removed as another liquid effluent fromthe process or also treated subsequently according to a variant of themethod which will be described later.

The next method comprises a step (d) of treatment of the solid resultingfrom the preceding step (second solid phase) optionally after washingthe latter with water.

This treatment is an acid attack. The solid can thus be dispersed inwater and an acid is added to the dispersion thus obtained.

The acid is preferably a strong acid such as nitric acid or hydrochloricacid.

The acid attack is carried out by adding the acid in conditions suchthat the pH of the reaction mixture is below 7, preferably below 3, forexample equal to 1.

According to an interesting variant, the dispersion or the mediumresulting from this acid attack is heated, for example up to its boilingpoint and it is held at this temperature and at the starting pH for aperiod of time which can for example be equal to 1 h.

This variant can be applied by autoclaving the dispersion or the mediumat a temperature which can be up to 200° C. for example.

The solid and liquid phases of the medium obtained at the end of acidattack are separated by any known means.

At the end of this separation, a third solid phase and a third liquidphase are collected. Washing of the solid phase with water can becarried out.

The liquid phase comprises at least one salt of one or more rare-earthelements and of the acid used for acid attack in step (d), for example anitrate or a chloride. This liquid phase constitutes the output of themethod of recovery of the invention since it contains the rare-earthelement or elements present in the solid starting mixture. In the caseof a mixture of rare-earth elements, these can be separated for exampleby known methods of the liquid/liquid extraction type.

The solid phase recovered contains all the species that were notattacked, for example alumina or silica, and it constitutes the solidresidue of the process.

According to a particular embodiment, which will now be described, themethod of the invention can comprise additional steps.

This embodiment is employed depending on the composition of the solidstarting mixture. Thus, it is particularly useful if this mixturecontains compounds such as aluminates of alkaline-earth metals and ofrare-earth elements and notably aluminates that comprise magnesium.

According to this embodiment, the method therefore comprises thefollowing additional steps:

(e) mixing the solid resulting from step (d) described above with asolid alkaline compound and calcining the mixture thus obtained;

(f) the calcined product from the preceding step is redispersed inwater;

(g) separating, from the dispersion obtained at the end of the precedingstep, a fourth solid phase comprising one or more rare-earth elements atleast in the form of hydroxide and a fourth liquid phase comprising atleast one alkaline element;

(h) preparing an aqueous dispersion of the solid resulting from thepreceding step and acidifying the dispersion thus obtained;

(i) separating, from the dispersion obtained at the end of the precedingstep, a fifth solid phase and a fifth liquid phase comprising at leastone rare-earth salt.

Step (e) is a step of solid/solid reaction (alkaline fusion) in whichthe solid resulting from step (d) is mixed with an alkaline compound,which can be for example a hydroxide, such as NaOH, an oxide such asNaO, and more particularly a carbonate, even more particularly a sodiumcarbonate. Mixing is carried out with a proportion by weight of alkalinecompound generally in excess relative to the solid to have an optimumreaction, and said proportion can be for example at least 3 parts ofalkaline compound to 1 part of solid, notably 5 parts of alkalinecompound to 1 part of solid.

Calcination is carried out in a furnace at a sufficient temperature toobtain fusion of the mixture, for example at a temperature of at least900° C. The duration of this calcination can be, just as an example,between 1 and 4 h.

At the end of calcination, the solid obtained is cooled and is dispersedin water, for example hot water, preferably with stirring, so as toobtain a dispersion.

The solid and liquid phases of the dispersion are then separated by anyknown means, notably by filtration, whereby a fourth solid phase and afourth liquid phase are obtained.

The solid phase, which can be washed with water, contains one or morerare-earth elements, which are generally present in the form ofhydroxides Ln(OH)₃, as well as residues of the alumina or silica type.

The liquid phase comprises an alkaline element which is that of thesolid compound used in step (e), this alkaline element generally beingin the form of a hydroxide for example NaOH, an aluminate for exampleNaAlO₂ or a silicate such as Na₂SiO₃.

This liquid phase can either be removed as liquid effluent from theprocess or further treated subsequently according to a variant of themethod which will be described later.

In order to recover the rare-earth element or elements contained in thesolid residue resulting from step (g) (fourth solid phase) this solid isdispersed in water and the dispersion thus obtained is acidified.

This acidification can be carried out with a strong acid, for examplewith nitric acid or hydrochloric acid. Acidification is carried out byadding the acid until a pH in the reaction mixture of less than 7 isobtained, preferably of at most 3, for example equal to 1.

According to an interesting variant, the dispersion or the mediumresulting from acidification is heated, for example up to its boilingpoint, and it is held at this temperature and at the starting pH for aperiod of time, which can for example be equal to 1 h.

The solid and liquid phases of the medium obtained are then separated byany known means, notably by filtration, whereby a fifth solid phase anda fifth liquid phase are obtained.

The solid phase recovered contains species that were not attacked, forexample alumina or silica.

The liquid phase also constitutes an output of the method of recovery ofthe invention, since it contains the rare-earth element or elements thatwere still present in the solid obtained at the end of step (d). In thecase of a mixture of rare-earth elements, separation of the latter canonce again be effected by known methods of the liquid/liquid extractiontype.

An interesting variant of the method of the invention will now bedescribed, which relates to the treatment of the liquid phases obtainedin the various steps that were described above.

According to this variant, the first and second liquid phases mentionedabove and, if applicable, the fourth liquid phase if steps (e) to (g)were employed, are mixed together. The pH of the mixture thus obtainedis adjusted to at least 5, more particularly at least 7, notably between7 and 8, whereby a precipitate is obtained in a sixth liquid phase, thenthe precipitate is separated from this sixth liquid phase.

The pH is adjusted to the value stated above by adding a basic or acidsolution to the mixture in relation to the respective amounts of thevarious liquid phases that were used to form the mixture.

The precipitate obtained can be mixed with the solid phase recovered atthe end of step (d) or step (i).

This variant can be particularly interesting when hydrochloric acid isused for the acid attack in step (a), a solution of soda is used in step(c) and if, when step (e) is applied, a compound of sodium is usedduring the latter. In this case, in fact a precipitate is obtained,which constitutes the solid effluent of the process and comprises ahydroxyapatite, and a sixth liquid phase which constitutes the liquideffluent of the process and which comprises sodium chloride. Thehydroxyapatite of type Ca₁₀(PO₄)₆(OH)₂ is a solid that can be utilized.

It should be noted that at the end of the method according to thisvariant, there is only recovery of a solid that can be utilized and aliquid effluent whose treatment does not pose any major problem, whichmakes this method advantageous from an environmental standpoint.

Non-limiting examples will now be given.

EXAMPLE 1

This example concerns a method according to the invention in whichnitric acid is used for the operations of acid attack.

As starting mixture, it starts from a powder of luminophores which has,per 100 g, the following composition expressed in oxides of the variouselements:

TABLE 1 Eu₂O₃ 1 g Gd₂O₃ 2 g CeO₂ 2 g Y₂O₃ 22 g  La₂O₃ 1 g Tb₄O₇ <1 g Al₂O₃ 7 g CaO 35 g  BaO 1 g MnO 1 g Sb₂O₃ 1 g P₂O₅ 27 g 

Attack Acid

In a stirred reactor, the powder of luminophores is dispersed in water,to obtain a dispersion with a concentration of 170 g/l, which is heatedto a temperature 70° C. A solution of nitric acid is poured onto thisdispersion, which is stirred and heated, in 1 h until the solution pHremains below 1. The mixture is then left to digest for 1 h, withstirring, and it is kept at the same temperature and at the same pH.

Then a solution of soda with a concentration equal to 4N is added in 1 hto the mixture, so that the pH of the mixture increases to 2. Themixture is stirred for 1 h.

After cooling, the mixture is filtered, and the filtration water isrecovered. It has the following composition:

TABLE 2 Al₂O₃   1 g CaO 25.8 g  MnO 0.5 g P₂O₅   5 g MgO 0.5 g SiO₂ 7.2g SrO 0.5 g ZnO 0.013 g  B₂O₃ 2.3 g Na₂O 11.6 g 

The residual solid, washed on the filter, is then recovered.

Attack with Soda

A soda solution at 55% and the preceding wet solid are added to astirred stainless-steel reactor. The amount of soda is such that theNaOH/solid weight ratio is equal to 3. The mixture is heated to atemperature of 140° C. for 45 min. Then after cooling to 80° C., themixture is diluted by adding hot water at 60° C.

Finally, after the mixture has cooled to 50° C., it is filtered, and themother liquor is recovered.

The solid residue is washed on the filter, then it is recovered and isredispersed in pure water in a stirred reactor.

Acid Attack

Then a solution of nitric acid is added, until a pH of 1 is obtained at60° C. The mixture obtained is filtered, and the mother liquor,containing the nitrates of rare-earth elements, is recovered for asubsequent treatment of separation of the rare-earth elements byliquid/liquid extraction.

The content of rare-earth elements expressed in oxide of this motherliquor is presented below.

TABLE 3 Eu₂O₃ 0.92 g Gd₂O₃  2.3 g CeO₂ 0.46 g Y₂O₃ 16.8 g La₂O₃ 0.46 gTb₄O₇ 0.46 g Total 21.4 g

Alkaline Fusion

The solid residue, after washing on the filter, is mixed with sodiumcarbonate Na₂CO₃, in a weight ratio of 5 parts of carbonate to one partof residue. The mixture, placed in a crucible, is introduced into afurnace for calcining at a temperature of 950° C. for 3 h. The fusedsolid obtained after cooling is disintegrated in hot pure water at 80°C. so as to obtain a dispersion of 100 g/l of starting residue, and isthen stirred for 1 h. After cooling, the dispersion is filtered, and themother liquor is recovered.

Dispersion/Acidification

The solid obtained, after washing on the filter, is dispersed in purewater. The dispersion is acidified by adding nitric acid, to pH 1, thenboiled for 1 h, keeping the pH at 1.

After cooling, the dispersion is filtered, and the mother liquor,containing the nitrates of rare-earth elements, is recovered forsubsequent treatment of separation of the rare-earth elements byliquid/liquid extraction.

The content of rare-earth elements expressed in oxide this mother liquoris presented below.

TABLE 4 Eu₂O₃ 0.22 g Gd₂O₃ 0.35 g CeO₂ 0.416 g  Y₂O₃ 1.85 g La₂O₃ 0.128g  Tb₄O₇ 0.22 g Total 3.184 g 

The overall yield for the rare-earth elements is 87.8%.

EXAMPLE 2

This example concerns a method according to the invention in whichhydrochloric acid is used for the first acid attack on the powder ofluminophore. The next steps of the method are carried out in the sameway as in example 1.

The tables given below show the analyses of the filtration waterrecovered at the end of the first acid attack (Table 5) and of thesecond acid attack (Table 6).

TABLE 5 Al₂O₃ 0.3 g CaO 29.4 g  MnO 0.73 g  Sb₂O₃ 0.012 g  P₂O₅ 16.2 g MgO 0.3 g SiO₂ 0.3 g SrO 0.2 g ZnO 0.022 g  B₂O₃ 2.1 g K₂O 1.5 g Na₂O12.51 g 

TABLE 6 Eu₂O₃ 0.85 g Gd₂O₃ 2.33 g CeO₂ 0.64 g Y₂O₃ 16.53 g  La₂O₃ 0.21 gTb₄O₇ 0.42 g Total 20.98 g 

The overall yield for the rare-earth elements is 84.1%.

1. A method of recovering rare-earth elements from a solid mixture comprising at least one halophosphate and at least one compound of one or more rare-earth elements, wherein the method comprises the following steps: (a) carrying out acid attack on said mixture in a liquid medium; (b) adding a base to the medium obtained at the end of step (a) so as to raise the pH of said medium to a value of at least 1.5, whereby a first solid phase is obtained comprising one or more rare-earth elements at least partly in the form of phosphate and a first liquid phase comprising at least one alkaline-earth element of halophosphate and separating a first solid phase from the first liquid phase; (c) attacking the first solid phase resulting from step (b) with a strong base, whereby a second solid phase is obtained comprising one or more rare-earth elements at least in the form of hydroxide and a second liquid phase comprising a phosphate of a cation of the strong base and separating the second solid phase from a second liquid phase; (d) carrying out acid attack on the second solid phase resulting from step (c) in conditions such that the pH of the reaction mixture is below 7, whereby a third solid phase is obtained and a third liquid phase comprising at least one rare-earth salt and separating the third solid phase from the third liquid phase.
 2. The method as claimed in claim 1, wherein a solid mixture is used that comprises, as a compound of a rare-earth element, a compound selected from the group consisting of phosphates, alkaline-earth aluminates, borates, vanadates and oxides.
 3. The method as claimed in claim 1, wherein the acid attack in step (a) is carried out with nitric acid or hydrochloric acid.
 4. The method as claimed in claim 1, wherein step (c) is carried out at a temperature of at least 100° C.
 5. The method as claimed in claim 1, wherein the method comprises the following additional steps: (e) mixing the third solid phase resulting from step (d) with a solid alkaline compound and calcining the mixture thus obtained; (f) redispersing, in water, the calcined product resulting from the preceding step; (g) separating, from the dispersion obtained at the end of the preceding step, a fourth solid phase comprising one or more rare-earth elements at least in the form of hydroxide and a fourth liquid phase comprising at least one alkaline element; (h) preparing an aqueous dispersion of the fourth solid phase resulting from the preceding step and acidifying the dispersion thus obtained; and (i) separating, from the dispersion obtained at the end of the preceding step, a fifth solid phase and a fifth liquid phase comprising at least one rare-earth salt.
 6. The method as claimed in claim 5, wherein a mixture comprising an alkaline-earth aluminate as a compound of a rare-earth element is used as a solid starting mixture.
 7. The method as claimed in claim 5, wherein after the acid attack in step (d) or the acidification, in step (h), the medium obtained at the end of these steps is heated, or is submitted to autoclaving.
 8. The method as claimed in claim 5, wherein the acid attack in step (d) or the acidification in step (h) is carried out with nitric acid or hydrochloric acid.
 9. The method as claimed claim 5, wherein the aforementioned first and second liquid phases and, if applicable, the fourth liquid phase are mixed together and the pH of the mixture thus obtained is adjusted to at least 5, whereby a precipitate is obtained in a sixth liquid phase, then the precipitate is separated from said sixth liquid phase.
 10. The method as claimed in claim 9, wherein the acid attack in step (a) is performed with hydrochloric acid, step (c) is performed with a solution of soda and, if step (e) is employed, a sodium compound is used during this step, whereby a precipitate comprising a hydroxyapatite and a sixth liquid phase comprising sodium chloride are obtained.
 11. The method as claimed in claim 7, wherein the medium obtained at the end of steps (d) or (h) is kept boiling. 